In recent years, demand has grown for miniature electric motors of extremely flat design, which can be used to build flat drives for disk-shaped data carriers such as floppy disks, CD-ROMs, DVDs etc.
The shaft of such a motor has a certain bearing clearance which allows the shaft to sway. Obviously, the sway angle of the shaft is the larger, the smaller the axial dimension of the bearing is (if the shaft is held by a single bearing) or the smaller the distance between bearings at opposite ends of the shaft is. If a disk is rotated by the motor, the axis of rotation of the disk may move in space, and its orientation may vary. This causes a problem when data are read from the disk: Conventionally, the data are arranged on concentric tracks on the surface of the disk, and if the axis of rotation of the disk moves, a servosystem of an actuated reading head of the disk drive may have more problems to follow the track during a turn of the disk, so that data cannot be read consecutively. The distance between the head and the surface of the disk may vary in addition and may sometimes leave the range in which reading is possible.
Part of these variations are periodic, with a period equalling the rotation period of the disk. These variations can be predicted quite accurately based on observation, and they can be compensated by moving the reading head periodically, so that it faces the same track of the disk and does not leave the proper reading distance range during a complete turn. These periodic variations are also referred to as repeatable runout.
However, there is also a non-repeatable runout which is not easily compensated and which may cause serious tracking problems. Reducing this non-repeatable runout by reducing the bearing clearance is a rather expensive solution to the problem, because the components of the motor have to be manufactured with very strict tolerances.
US 2004/0007929 A1 discloses a flat electric motor in which a bearing extends over most of the axial dimension of the motor. There is nothing in this motor that might prevent the shaft from swaying over the entire angle allowed by the bearing clearance.